1. Field of the Invention
The present invention relates to a method for producing a strained Si—SOI (Silicon-On-Insulator) substrate for a performance semiconductor device, and a strained Si—SOI substrate produced by the same. Specifically, the present invention relates to a technology suitable for use in a strained Si—SOI substrate, which improves the quality of an SOI wafer having a relaxed SiGe layer and a strained Si layer on an oxide film, that is, reducing the roughness, and decreasing the defects.
2. Description of Related Art
High speed current and low power consumption in silicon MOS devices have been realized compatibly, depending on the scaling law, either by minimizing the size of a device or by decreasing the operation voltage.
However, it is difficult to retain such compatible effects within a micro device having a gate length of less than 100 nm.
Accordingly, introduction of SOI substrates and strained silicon have been reviewed recently. Particularly, various investigations have been carried out on the production of SOI substrates having strained silicon introduced thereon as the most desirable substrate according to current technology.
As a first method, there is provided a combination of an SOI substrate and a SiGe by epitaxial technique.
For example, Patent Reference 1 describes a method in which a relaxed SiGe layer is formed by epitaxial growth of SiGe on an SOI substrate, and a layer of a strained Si is formed by the epitaxial grown of an Si layer on the relaxed SiGe.
As a second method, Patent Reference 2 describes a method for forming a strain relaxed SiGe layer on a buried oxide film using an SIMOX (Separation by Implanted Oxygen) method.
As a third method, Patent Reference 3 describes a method for relaxation of strain by forming a SiGe film on an SOI substrate, subsequently heat-treating the substrate under oxidizing atmosphere, and thereby causing downward diffusion of Ge, thinning of the SiGe layer, and Ge enrichment in the layer.
As a fourth method, Patent Reference 4 describes a method for strain relaxation of a SiGe film by forming a SiGe layer on an SOI substrate, melting the SiGe layer by heat treatment, and subsequently solidifying the SiGe layer while diffusing Ge in the layer.
As a fifth method, Patent Reference 5 describes a method for forming a Si—SOI substrate.
As a sixth method, a method for forming a strained silicon-SOI substrate in which only strained silicon exists in a buried oxide film utilizing a bonding method is described in Non-Patent Reference 1.    Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. Hei 07-169926    Patent Reference 2: Japanese Unexamined Patent Application, First Publication No. Hei 09-321307    Patent Reference 3: Japanese Unexamined Patent Application, First Publication No. 2000-243946    Patent Reference 4: Japanese Unexamined Patent Application, First Publication No. 2000-243946    Patent Reference 5: Japanese Unexamined Patent Application No. Hei 10-116473    Patent Reference 6: Japanese Unexamined Patent Application. First Publication No. 2004-363198    Non-Patent Reference 1: Extended Abstracts of the 2002 International Conference on Solid State Devices and Materials, pages 9 to 10 (ISSDM2002, Nagoya, 2002)
However, each of the above-described first to fifth methods are directed to a method for forming a relaxed SiGe layer on an insulating layer which is formed on a Si substrate, and forming strained Si on the SiGe layer. For example, in the case of using a buffer layer in which the Ge content is increased with a moderate gradient, generation of dislocations causes surface irregularities on the SiGe surface. Such surface irregularities reflect the distribution of dislocation lines and include lattice like steps called cross-hatches. Since the surface irregularities have an adverse effect on the photolithography which is carried out during a device-production process, it is necessary to remove the irregularities. Conventionally, a similar step for polishing the Si has been applied to the step for polishing the SiGe. However, in this case, the penetrating dislocation densities and the surface roughness of the formed SiGe layer have been insufficient, as compared with the desirable level for a device or a desirable level for raw materials for device production. Especially, the above-described cross-hatches are not uniformly distributed across the entire surface. At intervals of about several μm, the cross-hatches generate relatively large steps of about several tens of nm in height. It has been difficult to polish off such surface irregularities of the cross-hatches during polishing the SiGe using a general method for polishing the Si.
On the other hand, in the sixth method, only strained Si is formed on an insulating layer which has been formed on a Si substrate. However, the bonding method for making the strained Si—SOI substrate requires the epitaxial growth of a thick strained Si/SiGe layer and a plurality of steps including a bonding step, an exfoliation step, a film thinning step, and the like. Therefore, such a method has a disadvantage in that production cost is high.
In order to overcome such a problem, in Patent Reference 6, the Applicant disclosed a method for producing a semiconductor substrate having a Si substrate and a SiGe layer epitaxially grown on the Si substrate. The method comprises a film formation step of epitaxially growing a SiGe layer on a Si substrate; an oxide film formation step subsequent to the film formation step of the SiGe layer by oxidizing an upper surface of the SiGe layer; and an oxide film removal step subsequent to the oxide film formation step for removing the oxide film by etching (see, for example, Patent Reference 6). In this method as described in Patent Reference 6, after forming sequentially an amorphous SiGe layer having a predetermined Ge content and an amorphous silicon thin foil on an SOI substrate, hydrogen ions are implanted in the interface between the BOX (buried oxide) layer and the Si layer in the SOI substrate. The substrate is subject to heat treatment at least once at a predetermined temperature for a predetermined time under an oxidizing atmosphere. Subsequently, the substrate is subject to a heat treatment for melting the amorphous SiGe layer and the amorphous silicon layer. After the removal of the oxide film, strained silicon is formed.
However, in the above-described method, there is still a demand for further alleviation of the roughness of a new substrate surface, and the defects on the surface.
In accordance with the above-described circumstances, the present invention has been made. In this regard, it is an object of the present invention to provide a method for producing strained Si—SOI substrate having a flat surface and reduced defects. It is another object of the present invention to provide a strained Si—SOI substrate produced by the method.